Researchers believe the results have global relevance, with more than 91% of the world’s population breathing unsafe levels of air pollution.

Described as an invisible killer, air pollution causes an estimated seven million premature deaths a year worldwide, according to the World Health Organization.

“We provide evidence that the effect of air pollution on verbal tests becomes more pronounced as people age, especially for men and the less educated,” the study published on Monday in the Proceedings of the National Academy of Sciences said.

Pollution also increases the risk of degenerative diseases such as Alzheimer’s and other forms of dementia, the study suggests.

Exposure to high levels of polluted air “can cause everyone to reduce their level of education by one year”, explains one of the co-authors Xi Chen of the Yale School of Public Health to the Guardian, “which is huge”.

Previous studies found air pollution had a negative impact on students’ cognitive abilities.

In this study, researchers tested people of both sexes aged 10 and above between 2010 and 2014, with 24 standardised maths questions and 34 word-recognition questions.

Air pollution around the world – the stats

About 7m people die every year from exposure to polluted air

Air pollution caused 4.2m deaths globally in 2016

91% of the world’s population lives in places where air quality exceeds WHO guideline limits

Comparing people’s geographical location with when they were interviewed, allowed the researchers to match test scores with local air quality data.

One of the reasons they suggest older men with less education were worst affected by chronic exposure to air population is because those subjects often work outdoor manual jobs.

“Our findings about the damaging effect of air pollution on cognition,” the study concludes, “particularly on the aging brain, imply that the indirect effect on social welfare could be much larger than previously thought.”

The study suggests that while the research findings are specific to China, it can shed light on other developing countries with severe air pollution.

The authors point to the 98% of cities with more than 100,000 people in low- and middle-income countries that fail to meet WHO air quality guidelines.